This invention relates to flexible polyurethane foam. More specifically, this invention relates to very soft, low density, flexible polyurethane foam produced without using an auxiliary blowing agent. The foam formulations of this invention can be molded into polyurethane foam that is suitable for use as a substitute for polyester fiberfill in furniture applications.
Flexible molded polyurethane foam is recognized as the standard cushioning material for automotive seating. Molded polyurethane foam has not, however, succeeded in capturing much of the furniture market where polyester fiberfill is the cushioning material of choice due to its softness. For molded polyurethane foam to be considered as a substitute for polyester fiberfill in furniture applications, a very soft, low density, flexible polyurethane foam is needed. Prior attempts to modify the chemistry of auxiliary blowing agent free moldable polyurethane foam formulations to obtain soft, low density foams to replace polyester fiberfill have encountered myraid problems.
It is known in the art that to produce a polyurethane foam having the required low density and softness that a large amount of water needs to be incorporated into the foam formulation. It is also known that to obtain the softness required for furniture applications, it is necessary to run at low isocyanate indices. Running at low isocyanate indices, however, exacerbates the density problem, because at a low index the carbon dioxide produced by the water is reduced. Hence, the density reduction achieved with each additional part of water is reduced. Furthermore, as the amount of water is increased at lower isocyanate indices, the reaction of the water slows. As a result, water (probably in the form of steam) migrates to the surface of the foam mold where it condenses. This water on the mold surface subsequently reacts with isocyanate fumes and imparts a harsh, opaque, urea-rich surface to the foam part. Although this surface problem can be partially alleviated by further lowering the isocyanate index, doing so results in higher compression set properties and slower demold times. Increasing the mold temperature would probably help, but is not feasible due to the operating temperature ranges of commercial mold release agents. In addition to the harsh surface problem described above, prior attempts to produce the desired very soft, low density molded polyurethane foams have resulted in unstable foams and in foams which exhibit a significantly higher propensity to "hot" crusher set.
U.S. Pat. Nos. 4,883,825 and 4,421,872 teach flexible molded polyurethane foam formulations which yield low density foams. However, these foams are automotive seating foams and are much too firm for use as a substitute for polyester fiberfill in furniture applications. More specifically, U.S. Pat. No. 4,883,825 teaches a process for the manufacture of low density flexible polyurethane foam via the reaction of a highly reactive polymer polyol containing a high primary hydroxyl content, a hydrophilic polyhydric compound, and water, with a polyisocyanate. The process produces low density, rapid demold systems for high resilience molded foam parts such as automotive seating. These foams however are much firmer than the foams of this invention and are not suitable for use as a substitute for polyester fiberfill in furniture and bedding applications. U.S. Pat. No. 4,421,872 teaches the use of a highly reactive polyol for the manufacture of flexible polyurethane foam. Also taught is the use of crosslinkers/chain extenders such as diamines and short chain diols as a way to increase foam firmness. The resulting foams are not very soft and, accordingly, are not suitable to replace polyester fiberfill in furniture and bedding applications.
The present invention overcomes many of the problems encountered in the prior art by providing a new polyurethane foam formulation which facilitates the molding of very soft, low density, flexible polyurethane foam. No auxiliary blowing agent, such as methylene chloride or a halogenated hydrocarbon, is required in the foam formulation of this invention.